The perioral route for drug administration remains the most convenient way of clinical therapy and is preferred by patients, however, good bioavailability is a necessary characteristic of new candidate therapeutics. A key parameter for determining oral bioavailability is drug transport and permeability across the small intestine (SI) epithelium. Therefore, in vitro models of SI epithelium that accurately predict in vivo human transport/permeability of candidate drugs are of high interest to the pharmaceutical industry. Currently available in vitro models have significant deficiencies which limit their utility for dru development applications, including poor correlation to in vivo human carrier-mediated transport and par cellular permeation, as well as a lack of in vivo human small intestine metabolic enzyme activity. The current proposal aims to develop an in vitro human SI for use in pharmaceutical development applications. The SI model will consist of primary human SI epithelial cells and fibroblasts cultured on micro porous membrane supports to produce well-differentiated SI tissues with in vivo-like human drug transport and permeability characteristics. Because the model is produced from primary normal (non-cancerous) SI cells, the tissue is expected to provide superior performance compared to currently available models based on animal or cancer-derived cell lines. Significant preliminary results indicate that this goal is readily attainable. Aims of the project include optimization of culture parameters, characterization of drug transporters and metabolizing enzymes, and permeation studies using a number of substrates that have been previously studied using the industry standard model, Caco-2 cells. Drug permeation via passive diffusion and active transport, along with the effect of transporter inhibitors, will be studied. A comparison between the permeation/absorption in: a) the SI model vs. human and b) Caco-2 cells vs. human will be made to determine if the SI model offers improved pharmacokinetic predictions versus the Caco-2 model. Phase 2 works will include further optimization of the SI tissue model to a 96-well, high-throughput screening format, transfer of the in vitro method to other laboratories, and performance of a blinded validation study in multiple laboratories. Results will be submitted to the US Interagency Coordinating Committee on Validation of Alternative Methods (ICCVAM) for review and validation. In addition, the effect of alternative donors and various pathological conditions on SI permeation will be investigated. The availability of an accurate, cost-effective, and reproducible in vitro means of assessing SI drug absorption will enhance the drug selection process and improve the probability of clinical success of new drug candidates. This novel, in vitro tool will find signifiant commercial acceptance throughout the pharmaceutical industry.